Emergency locating transmitters (ELTs) are required by federal law on all aircraft travelling more than twenty-five miles from an airport, and emergency position indicating radio beacons (EPIRBs) are required on certain classes of marine vessels. ELTs and EPIRBs are essentially the same device. They differ only in the manner in which they are activated. ELTs are activated by gravitational forces (impact) while EPIRBs are activated by water contact. Both, however, may be manually activated.
Prior art ELTs and EPIRBs transmit a distress waveform on 121.5 MHz and 243 MHz to alert emergency frequency monitors that a distress incident has occurred. The distress waveform transmitted by these devices consists of an amplitude modulated carrier signal in which the modulating signal is an audio frequency sweeping downward over a range of not less than 700 Hz, within the range of 1,600 Hz to 300 Hz, and at a sweep rate varying between 2 Hz and 4 Hz. These characteristics are required by federal law, so that the transmitted distress waveform, which can be demodulated by a receiver to a siren-like sound, may easily be recognized by individuals monitoring on 121.5 MHz and 243 MHz, who can then alert search and rescue (SAR) personnel to search for the location of the source of the distress transmission and initiate rescue operations. The distress waveform, however, does not contain information other than that an ELT or an EPIRB is transmitting it. Accordingly, SAR personnel receive no advance information on whether they are searching for an airplane, marine vessel, camper, hiker, or skier. This uncertainty contributes to the inefficient use of SAR personnel and in poor coordination among rescue operations.
The siren-like distress waveform has been designed to be primarily detected by a radio operator. However, a relatively strong signal level of at least 30 db-Hz is required before a radio operator can distinguish the siren-like sound from the background noise. This is because the frequency spectrum of some distress waveforms closely approximates the frequency spectrum of the background noise, and accordingly the siren-like sound will sound like noise until the threshold level of 30 db-Hz is reached. The close approximation of the frequency spectrum of the distress waveform to the frequency spectrum of the background noise also makes it difficult to electronically distinguish the distress waveform from the background noise.